Other than MPI_COMM_SPAWN[_MULTIPLE], we don't expose the underlying run-time to MPI applications.

There is a whole programatic interface for the layer under MPI (the Open MPI Runtime Environment -- ORTE), though. We don't advise mixing ORTE calls in MPI applications, but it is certainly feasible to use ORTE for non-MPI things (some of the OMPI community member organizations do so).